Category: MSLP

You can tell just by looking at these sea surface temperature charts since 2015 (figs 1-3) that there’s been considerable warming going on in the last two years. The cold anomaly that was a feature for so long in the central Atlantic is now less intense and much smaller a feature than it once was. It’s shifted and has been squeezed further northeast towards the southeast coast of Greenland and the opening of the Labrador Sea. The other notable difference that’s appeared in recent months is the area of warmer SST anomalies ~1500 km to the west of Portugal (42N 17W), a warm blob if you like.

The warm blob and this coming winter

To be honest I don’t have a clue if this warm blob will have the slightest effect on the weather in the British Isles this coming Winter. It would be nice to think that the area of warmer SST might weaken the Azores high in some way, and that might increase cyclonic development in the southeast Atlantic which will run northeastward towards Biscay – who knows. Looking at the actual MSLP anomalies (fig 3) for so far this month, it’s true that the Azores high is displaced further to the northeast, but all this has done has been to increase the strength of the zonal flow across the Atlantic. In fact everything seems to be enhanced in some way, if you look at the underlying anomalies (red dashed line) both highs and lows have been more intense in the first three weeks of November 2017 than usual.

I’ve just downloaded the latest NCEP reanalysis data and run the stats for MSLP across the world, and you can see why the first half of October 2017 has been so mobile, and relatively mild over the British Isles, by studying the latest mean pressure and anomaly chart for the North Atlantic (fig 1). The MSLP anomalies across the North of Scotland are around 6 hPa below the 1948-2014 long-term average, whilst across southern England they are 1 hPa above.

Figure 2

In contrast a belt of higher than average pressure extends right from the western Atlantic, across America and into the central Pacific, and highlights the reasons behind the recent devastating forest fires across northern California (fig 2).

So why have temperatures in the first half of September 2017 been so cool across most of northwest Europe? One glance at the mean pressure chart for the first 16 days will give you the answer (fig 1). The Azores high has been +5 hPa stronger than average, and the Icelandic low has become elongated eastward, with pressure 10 hPa lower than average across the northern Isles. This has resulted in a strong west northwesterly flow across the central Atlantic across much of central Europe. I’m sure that the fast-moving ribbon of air which some people call the jet stream has something to do with it, I find that it usually does.

Figure 1 – Data courtesy of NCEP reanalysis

Here are the fine details of how the circulation has been behaving over the last couple of years with the help of some values from the objective LWT analysis (fig 2). I’ve highlighted September to show how strong and persistent the combined SW-W-NW theme has been this month.

Figure 2

It’s quite noticeable, that from the from the third week in July, maximum temperatures in the CET series have generally been rather flat and slightly below average (fig 3). If you look at the spells bar chart (the fifth chart down) there have been few if any prolonged warm or cold spells longer than 3 days or more with anomalies 2°C either above or below the long-term average, compared with previous summers.

Figure 3 – Courtesy of NCEP reanalysis

Looking over a much larger area with the 12 UTC mean temperature anomalies for the first 16 days of September, you can see that the increased westerly flow has resulted in a large -3°C temperature anomalies across central Europe, and even higher +5°C warm anomalies over northeast Turkey (fig 4).

Figure 4

What about the coming Autumn and Winter?

What does all this portend for the coming Autumn and Winter? I have got absolutely no idea. But if this kind of strong anomalous west or northwesterly flow continue like it’s been doing, I would guess that it looks likely to be cold, windy and rather wintry at times.

I don’t know if the 937.6 hpa read at Stornoway at 0020 UTC on the 20th of December 1982, was indeed the lowest minimum pressure of the 20th century recorded in the UK, but it was certainly extremely low. This is the midnight chart that I have reassembled from the old SYNOP reports (fig 1).

The pressure has fallen surreptitiously and steadily over the last few days here in the southwest, from around 1028 to 992 hPa without most of the population probably even noticing, unless of course they still have a hall barometer that they tap before going out to work, but that’s my very old-fashioned 1950’s suburban view of the world, that’s all but gone in these days of smartphones and tablets.

That fall of pressure and the recent dry spell, got me thinking about the obvious close correlation that exists between air pressure and precipitation, so I thought that I would overly the pressure and precipitation in a single graph, and hey presto a new graph, which I call a baro-hyetograph for want of a better word, is born. The top baro-hyetograph is for the period since the start of the year for Exeter airport, and shows the recent dry spells and anticyclonic periods very well (fig 1). The second baro-hyetograph is for Heathrow airport and paints a very similar picture (fig 2).

You may wonder why I can produce a fairly accurate barograph for the last seven months for some obscure farm in the heart of rural Shropshire. The reason why I can is that the six-hour reanalysis MSLP data that I download from the NOAA 20th century reanalysis project is based on a 2.5° x 2.5° grid, and Stockings farm is little more than a few hundred yards from latitude 52.5° north and longitude 2.5° west (fig 1).

Figure 2

The barograph (fig 2) shows the anticyclonic nature of last Autumn and Winter, the pink vertical bars are the five named storms that occurred during that period. I thought that I’d made a mistake when checking the results around Christmas, thinking that the anomalies were positive and much too high, but then I realised that the storm force winds from storm Barbara and Conor were caused as much by the reluctance of pressure to give way further south, as it was by the low pressure from the vigorous extratropical cyclones crossing the Norwegian sea further north. This did not happen with storm Angus, Doris or Ewan.

I’ve just been looking at the mean pressure chart for April 2017, and it was certainly an anticyclonic month with a large +12 hPa anomaly to the west of Ireland which produced a sizeable 1025 hPa anticyclone at 50°N 12°W, and a northwesterly flow across the British Isles. This made April 2017 a very dry and rather mild month across the British Isles, which although cloudy in the far west, was rather sunnier than average elsewhere. April 2017 does look to have a fairly strong resemblance to the April’s of 1967, 1980, 1982, 1995, 2015 and 1997.